mm/cow: don't bother write protecting already write-protected pages

torvalds · torvalds · commit 1b2de5d039c8 · 2018-08-25T13:15:03.000-07:00
This is not normally noticeable, but repeated forks are unnecessarily expensive because they repeatedly dirty the parent page tables during the page table copy operation. It's trivial to just avoid write protecting the page table entry if it was already not writable. This patch was inspired by https://bugzilla.kernel.org/show_bug.cgi?id=200447 which points to an ancient "waste time re-doing fork" issue in the presence of lots of signals. That bug was fixed by Eric Biederman's signal handling series culminating in commit c3ad2c3 ("signal: Don't restart fork when signals come in"), but the unnecessary work for repeated forks is still work just fixing, particularly since the fix is trivial. Cc: Eric Biederman <ebiederm@xmission.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memory.c b/mm/memory.c
@@ -1022,7 +1022,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	 * If it's a COW mapping, write protect it both
 	 * in the parent and the child
 	 */
-	if (is_cow_mapping(vm_flags)) {
+	if (is_cow_mapping(vm_flags) && pte_write(pte)) {
 		ptep_set_wrprotect(src_mm, addr, src_pte);
 		pte = pte_wrprotect(pte);
 	}

Original file line number	Diff line number	Diff line change
`@@ -1022,7 +1022,7 @@ copy_one_pte(struct mm_struct dst_mm, struct mm_struct src_mm,`
`1022`	`1022`	`* If it's a COW mapping, write protect it both`
`1023`	`1023`	`* in the parent and the child`
`1024`	`1024`	`*/`
`1025`		`- if (is_cow_mapping(vm_flags)) {`
	`1025`	`+ if (is_cow_mapping(vm_flags) && pte_write(pte)) {`
`1026`	`1026`	`ptep_set_wrprotect(src_mm, addr, src_pte);`
`1027`	`1027`	`pte = pte_wrprotect(pte);`
`1028`	`1028`	`}`